The present invention relates to a method for polishing a work and an apparatus for polishing a work, more particularly to a method for polishing a plane portion of a work, such as a surface of a semiconductor wafer, with a polishing pad. More particularly, the present invention relates to a method of releasing a work from a polishing pad after polishing and a polishing apparatus for conducting the method.
Conventionally, an apparatus equipped with work holding plates made of metal, ceramics, glass or the like which are attached respectively to rotary a plurality of holders which can rotate and a polishing pad attached to a polishing turn table which can rotate is known as an apparatus for polishing a surface of a work, such as a semiconductor wafer.
The method for polishing a surface of a work using such a polishing apparatus is generally conducted as follows. The side opposite to the surface to be polished of each of the works is attached to a work holding surface of each of the work holding plates so that the work is held, which are then moved relatively to the polishing pad so that the plural works are polished at the same time, with pressing the side to be polished of the work on the polishing pad and with supplying polishing liquid on the polishing pad.
There are various methods for holding a work, including a method wherein one or more wafers are adhered to a work holding plate with wax, or a method of holding a wafer by vacuum-sucking to a work holding plate.
A method of polishing using a four axes polishing apparatus equipped with four work holding plates will be explained hereinafter in more detail. Predetermined number of works are held on each of the work holding plates, they are polished at the same time by rotating both a polishing turn table and all of the work holding plates, with supplying polishing liquid on a polishing pad and with pressing four works on the polishing pad simultaneously. In this case, in order to press the surface to be polished of the work on the polishing pad by predetermined pressure, in addition to the pressure by its own weight of the work holding plate or the like, compulsory pressure (this pressure applied compulsorily is called press pressure) is suitably applied between the surface to be polished of a work and a polishing pad through a work holding plate by a pressure applying means. After polishing the work surface to a desired polishing amount, press pressure of each of four works to a polishing pad is set about 0 simultaneously, to terminate polishing substantially. Subsequently, the work holding plates are pulled up, and the works are released from the polishing pad, and further the works are removed from a holding plate.
For example, when a work is a semiconductor wafer, the wafer polished and released from the polishing pad as described above is usually transferred to the following cleaning process. In many cases, the work is kept in pure water or a solution wherein ozone, hydrogen peroxide, or the surface-active agent is added to pure water until it is transferred to the cleaning process after polishing process. It is conducted in order to remove the polishing liquid adhered to the wafer surface after polishing. Furthermore, if the polished wafer is left as it is in the atmosphere, particles adhered to the wafer surface at the polishing process are dried and stuck thereto, and it is difficult to remove them by cleaning in the following process. Accordingly, it is temporarily kept in pure water or the like as described above.
Removal of the polishing liquid or particles adhered to the surface as described above is necessary not only after a semiconductor wafer is polished, but also after any works are polished.
As described above, after the works held on the work holding plate are simultaneously pressed on the polishing pad and polished, to a desired polishing amount, the applied pressure is set about 0 simultaneously to terminate polishing. At this time, the work is in contact with the polishing pad and is left on the polishing pad. Subsequently, the work is released from the polishing pad by pulling up the work holding plate further. The works are not released from the polishing pad simultaneously, but are released from the polishing pad one by one.
Accordingly, time until the work is released from the polishing pad after termination of polishing (hereinafter referred to as xe2x80x9cleaving timexe2x80x9d) varies depending on works. Evaluation of each surface of each work with a particle counter revealed that there was a tendency that on the wafer left on the polishing pad after polishing for longer time, the number of counts was larger.
Generally, the defect represented as particles by a particle counter using optical measurement is called LPD (abbreviation for Light Point Defect), which includes, in addition to particles adhering to the work surface, depression formed as a result of corrosion of the defect on the work surface caused by a chemical action of a polishing liquid (hereinafter referred to as pit), and other form irregularities or the like which may cause optical scattering. Therefore, the measured value represents the sum of them.
Usually, polishing of a semiconductor wafer, such as silicon wafer is conducted by chemical mechanical polishing (CMP) which is combination of the mechanical polishing action by the polishing particle or the like and the chemical polishing action by the substance having an etching action for works, such as alkali, amines or the like, in order to prevent the damage from remaining on a polished side. Therefore, a chemical action with polishing liquid can be uneven etching with the substance having an etching action.
Accordingly, it is considered that the reason why the count number measured with the particle counter is larger when the leaving time of the wafer is longer, may be that fine particles such as polishing particles contained in a polishing liquid remaining and adhered on the polished surface of the work are increased, that local corrosion of the polished surface of the work by the chemical action of the polishing liquid progresses with time and the number of fine depressions (pits) increases, or both of them.
It is possible to remove the fine particles adhering to the surface of the polished work by cleaning. However, in order to clean the plural works at the same time and to remove particles from all of the works, it is necessary to clean the works so that a work to which the largest amount of particles are adhered may be cleaned, and cleaning for a long time must be performed after all.
Such long time cleaning causes, in addition to the problem that cost is raised as a result of using a lot of cleaning liquid, a disadvantage that efficiency is low, since a work to which a little amount of particles are adhered is cleaned for a unnecessarily long time.
Moreover, in the work to which too many particles are adhered, it is difficult to remove them by usual cleaning, which may cause the problem of contamination by particles in a product after all.
The reason why a difference appears in the value of LPDs depending on the leaving time after termination of polishing until releasing of the work from a polishing pad is not certain. However, in a multi axes apparatus, while the number of works that can be processed (polished) at once is increased, but it comes to require time until all works are collected in a storage tank. It is thought that the wafer surface is very active immediately after processing and is easy to receive contamination by the metal ion or the foreign substance, and therefore, the particle level of the wafer gets worse as the time necessary for collection of the wafer becomes long.
Among of them, the most serious problem is increase of pits as a result of corrosion with polishing liquid. Once they are formed on the surface of a work, they cannot be removed by cleaning, and thus lowering of the quality of the work cannot be avoided.
The present invention has been accomplished to solve the above-mentioned problems, and an object of the present invention is to provide a method for polishing a work wherein formation of pits on the surface of a work due to polishing liquid can be prevented, an amount of the cleaning liquid used in a cleaning process can be reduced, particles can be removed efficiently in a short time, the value of LPD detected on the surface is low, and there is almost no deviation between works, and a work polishing apparatus therefor.
In order to achieve the above-mentioned purpose, the present invention provides a method for polishing a work wherein polishing liquid is supplied to a polishing pad, and a relative movement is carried out between the work and the polishing pad with pressing the work on the polishing pad, characterized in that the work of which press pressure was set to 0 to terminate polishing is released from the polishing pad within 45 seconds after the termination of polishing.
As described above, concerning the defect on the surface of a work, since formation of pits is caused by corrosion due to the polishing liquid, it is desirable to release the work from the polishing pad as early as possible after polishing.
The inventors of the present invention investigated the relation between time until a work is released from a polishing pad after polishing and the number of counts on the work surface measured with the particle counter, and found that if the work was released from the polishing pad within 45 seconds after polishing was terminated by setting press pressure of the work to 0 as mentioned above, the number of counts measured by the particle counter could be controlled to be small, and thus it was satisfactory in quality. Namely, by releasing the work from the polishing pad within 45 seconds as described above, the corrosion on the surface of the work due to the polishing liquid on the polishing pad can be suppressed, and LPD due to pits can be suppressed.
Moreover, the amount of the particles adhered on the surface of a work can also be decreased by releasing the work after polishing from a polishing pad in a short time of 45 seconds or less as described above. Therefore, an amount of the cleaning liquid used in a cleaning process can be reduced, and particles can be removed surely and efficiently in a short time.
In the present invention, xe2x80x9csetting press pressure to 0xe2x80x9d means canceling compulsory force of the work held on the work holding plate against the polishing pad. The load by weight of the work itself or the ceramic plate or the like to which the work is adhered is not included in the press pressure as used in the present invention.
Moreover, according to the present invention, in the polishing method wherein plural works are simultaneously polished by relative movement between the works and a polishing pad with pressing the works on the polishing pad and supplying the polishing liquid to the polishing pad, wherein only the work of which press pressure was set to 0 to terminate polishing is released from the polishing pad within 45 seconds, while polishing of the other works can be continued.
As described above, if only the work of which press pressure was set to 0 to terminate polishing is released from the polishing pad, and the other works are kept to be polished, the work which had been polished as described above can be released from the polishing pad within 45 seconds, corrosion due to the polishing liquid of the surface can be suppressed, and only few amount of particles are adhered thereon. The other works are not left on a polishing pad unlike in a conventional method, since these works are kept to be polished.
In some cases, the so-called water polishing supplying only water may be performed after stopping supply of the polishing liquid. Also in this case, increase in pits of the surface occurs due to the remaining polishing liquid, and thus it is desirable to release the work within 45 seconds after pressing of a work is stopped.
Moreover, when a work is a semiconductor wafer, polishing is often conducted in several steps. The polishing method of the present invention is effective for polishing in each step. However, in the stepwise polishing, the amount of polishing is fewer in the latter step, and therefore, influence with polishing liquid needs to be suppressed more in the latter step. Therefore, the present invention is especially effective for polishing in the last step in which the amount of polishing is the fewest, namely so-called finish polishing.
Furthermore in this case, it is desirable that the press pressure of the plural works is set to 0 in turn so that the work is released from the polishing pad.
Namely, only the work of which press pressure was set to 0 to terminate polishing is released from the polishing pad within 45 seconds after polishing, whereas the other works are kept to be polished. Subsequently, polishing of either of the works which are kept to be polished is terminated one by one or simultaneously. Then, only the work after termination of polishing is released from the polishing pad within 45 seconds after polishing, whereas the other works are still kept to be polished. When the press pressure of plural works is set to 0 to release the work from the polishing pad in turn, one set of works of which press pressure against the polishing pad are simultaneously controlled may be released as a group, namely group by group.
The remaining works which are being polished are also released from the polishing pad one by one according to the above-mentioned procedure. By releasing all works from the polishing pad within 45 seconds after polishing, there can be obtained works wherein corrosion due to the polishing liquid on the work surface is suppressed, only few particles are adhered, LPD value is significantly small, and there is little variation between the works.
Furthermore, the present invention provides a polishing apparatus for works having at least one or more work holding plates for holding works and a turn table on which a polishing pad is fixed, and polishing plural works simultaneously by relatively moving the works and the polishing pad with pressing the works held on the work holding plate to the polishing pad and with supplying the polishing liquid to the polishing pad, characterized in that each of the work holding plates has a function of setting press pressure to 0 to terminate polishing independently and releasing the work after polishing from the polishing pad within 45 seconds.
As described above, if a work is polished using a work polishing apparatus equipped with not only single work holding plate but also plural work holding plates having a function of setting press pressure to 0 to terminate the respective polishing independently and releasing the work after polishing from the polishing pad within 45 seconds, each of the works held on the work holding plate can be released one by one from the polishing pad within 45 seconds after termination of polishing, and thereby there can be obtained works having excellent surface quality wherein corrosion due to polishing liquid can be suppressed, only few amounts of particles are adhered, and thus LPD value is significantly small, and there is no variation between the works.
As described above, the present invention relates to a method for polishing works wherein polishing liquid is supplied to a polishing pad, and a relative movement is carried out between the work and the polishing pad with pressing the work on the polishing pad, characterized in that the work of which press pressure was set to 0 to terminate polishing is released from the polishing pad within 45 seconds after the termination of polishing. As described above, the work released from the polishing pad in a short time after termination of polishing as described above has almost no pits formed as a result of corrosion due to polishing liquid, and has excellent surface quality. Furthermore, only few amounts of particles are adhered, and there is no deviation in the works. Accordingly, an amount of the cleaning liquid used in a cleaning process can be reduced, and particles can be removed efficiently and certainly in a short time. Thus, a work having an excellent surface quality can be surely obtained.